Doctor of Philosophy
Fast radio bursts (FRBs) are short and extremely energetic bursts of radiation detected from galaxies across the universe that occur thousands of times a day. Despite advances in instrumentation, it is difficult to explain the enormous implied energy reservoirs of FRBs, their emission mechanism and the existence of repeating and periodic sources. This thesis explores the spectro-temporal properties of repeating FRBs and details the discovery of several new relationships between them, providing valuable information on the nature of FRBs. By measuring the spectro-temporal properties of a sample of bursts from the repeating source FRB20121102A I show that the magnitude of a burst's time derivative of the frequency (or ``sub-burst slope") is inversely proportional to its duration. This relationship is a key prediction of the triggered relativistic dynamical model (TRDM), a model that assumes FRBs are inherently narrow-band in nature and originate from a cloud of material. I then investigate other FRB sources by analysing bursts from the repeaters FRB20180814A and FRB20180916B, discovering that the same slope-duration relationship describes the bursts from all three sources. Because FRBs are subject to dispersion by free ions along the line of sight, and because the measurement of spectro-temporal properties are dramatically affected by the choice of dispersion measure (DM), measurements of each burst are performed over a range of DMs to estimate uncertainties and validate any relationships found. Finally, I developed a software tool for preparing and measuring properties of FRBs and used it to survey a broad sample of 167 bursts from FRB20121102A. This sample spans 1 to 7 GHz, the entire range of burst frequencies observed for this source. I find relationships between a burst's duration, slope, and frequency consistent with the TRDM, and discover an unexpected relationship between the bandwidth of a burst and its duration. These spectro-temporal relationships can be important tools and suggest a narrow-band emission mechanism for FRBs.
Summary for Lay Audience
The sky can be observed in colors far beyond what our own eyes can see. At radio frequencies, radio telescopes can detect short bursts that come from extreme environments such as neutron stars. Fast radio bursts (FRBs) are one such type of burst discovered in 2007. Extremely bright and coming from distant galaxies, these bursts were difficult to discover because they are shorter than a few milliseconds and arrive at Earth distorted by the material between galaxies. This distortion is caused by electrons and other ions and is similar to the dispersion of white light by a prism into a rainbow. After correcting for this dispersion astronomers can study FRBs but to this day struggle to explain why there are thousands per day, where they come from, and how they can be bright enough to be seen from galaxies far beyond our own. Some sources of FRBs are also seen to repeat, and some of those still are periodic and follow a known schedule.
Telescopes today can split up the frequencies that make up an FRB like a prism does while measuring its brightness with time. With this information, we can make two-dimensional images of each burst called a waterfall. I study the shapes FRBs make in their waterfalls and use this information to understand what might be causing them. Bursts often appear as lines in their waterfalls, and I find that the steeper that line is, the shorter the burst. Patterns like this help us to understand what is happening, and by using physical theories like relativity we find we can explain the shapes FRBs make in their waterfalls. The patterns I have found are most easily explained if FRBs come from a cloud of atoms and are emitted at a single narrow frequency. These clouds could be triggered by a small dense object like a neutron star. The high speeds of these clouds, close to the speed of light, shift the frequency of that narrow signal and create the many waterfall shapes we later see on Earth.
Chamma, Mohammed Afif, "The Spectro-Temporal Relationships of Repeating Fast Radio Bursts" (2022). Electronic Thesis and Dissertation Repository. 8958.
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